This invention relates to the preparation of trichlorosilane for the manufacture of electronic grade silicon and, more particularly, to novel methods for removing trace impurities of electrical donor contaminants, especially phosphorus and other Lewis base or proton acceptor-type impurities.
Silicon of extremely high purity is required for sophisticated electronics uses such as in semiconductors and transistors. It is well known that even trace impurities can seriously impair the performance of silicon-containing electronic components.
Elemental silicon for semiconductor use is generally prepared by reduction of silicon halides, such as silicon tetrachloride (SiCl.sub.4), trichlorosilane (HSiCl.sub.3) and dichlorosilane (H.sub.2 SiCl.sub.2), with hydrogen, zinc, sodium or metal hydrides. Silicon may also be derived from thermal decomposition of silane (SiH.sub.4), but this latter material is hard to work with because it burns explosively on contact with air.
One of the most difficult impurities to remove from high purity silicon is phosphorus. Whereas other impurities such as copper, iron and manganese are comparatively easy to remove by conventional techniques (e.g., zone refining, crystal pulling), phosphorus has physical properties so similar to silicon that separation is accomplished only by repeated trials. Moreover, concentrating purification efforts on the starting materials, e.g., chlorosilanes, is likewise difficult because phosphorus forms corresponding compounds with similar properties.
Previously proposed methods for the removal of phosphorus and like impurities from silicon or silicon halide materials typically involve adsorption of impurities by contact with solid, hydrated oxymetal complexing agents, or formation of stable addition compounds followed by deposition or distillation of pure silicon or silicon halide. U.S. Pat. Nos. 2,971,607 (Caswell), 3,069,239 (Winter et al.), 3,071,444 (Theuerer), 3,188,168 (Bradley) and British Pat. No. 929,696 (Siemens-Schuckertwerke Aktiengesellschaft), for instance, describe such treatments. Improved processes are also disclosed in commonly assigned, copending U.S. application Ser. Nos. 449,763 and 439,783, filed Dec. 14, 1982 and Nov. 8, 1982, respectively. However, problems with regeneration of impurities and lack of adaptability to large-scale use are experienced with some of these methods.